The prior art includes:                1. Daniel Sievenpiper, U.S. Pat. No. 6,496,155        2. P. W. Chen, C. S. Lee, V. Nalbandian, “Planar Double-Layer Leaky Wave Microstrip Antenna”, IEEE Transactions on Antennas and Propagation, vol. 50, pp. 832-835, 2002        3. C.-J. Wang, H. L. Guan, C. F. Jou, “Two-dimensional scanning leaky-wave antenna by utilizing the phased array”, IEEE Microwave and Wireless Components Letters, vol. 12, no. 8, pp. 311-313, 2002        4. J. Sor, C.-C. Chang, Y. Qian, T. Itoh, “A reconfigurable leaky-wave/patch microstrip aperture for phased-array applications”, IEEE Transactions on Microwave Theory and Techniques, vol. 50, no. 8, pp. 1877-1884, 2002        5. C.-N. Hu, C.-K.C. Tzuang, “Analysis and design of large leaky-mode array employing the coupled-mode approach”, IEEE Transactions on Microwave Theory and Techniques, vol. 49 no. 4, part 1, pp. 629-636, 2001        6. E. Semouchkina, W. Cao, R. Mittra, G. Semouchkin, N. Popenko, I. Ivanchenko, “Numerical modeling and experimental study of a novel leaky wave antenna”, Antennas and Propagation Society 2001 IEEE International Symposium, vol. 4, pp. 234-237, 2001        7. J. W. Lee, J. J. Eom, K. H. Park, W. J. Chun, “TM-wave radiation from grooves in a dielectric-covered ground plane”, IEEE Transactions on Antennas and Propagation, vol. 49, no. 1, pp. 104-105, 2001        8. Y. Yashchyshyn, J. Modelski, “The leaky-wave antenna with ferroelectric substrate”, 14th International Conference on Microwaves, Radar and Wireless Communications, MIKON-2002, vol. 1, pp. 218-221, 2002        9. H.-Y. D. Yang, D. R. Jackson, “Theory of line-source radiation from a metal-strip grating dielectric-slab structure”, IEEE Transactions on Antennas and Propagation, vol. 48, no. 4, pp. 556-564, 2000        10. A. Grbic, G. V. Eleftheriades, “Experimental verification of backward wave radiation from a negative refractive index metamaterial”, Journal of Applied Physics, vol. 92, no. 10        11. J. W. Sheen, “Wideband microstrip leaky wave antenna and its feeding system”, U.S. Pat. No. 6,404,390B2        12. T. Teshirogi, A. Yamamoto, “Planar antenna and method for manufacturing same”, U.S. Pat. No. 6,317,095B1        13. V. Nalbandian, C. S. Lee, “Compact Wideband Microstrip Antenna with Leaky Wave Excitation”, U.S. Pat. No. 6,285,325        14. R. J. King, “Non-uniform variable guided wave antennas with electronically controllable scanning”, U.S. Pat. No. 4,150,382        
The presently disclosed technology relates to an electronically steerable leaky wave antenna that is capable of steering in both the forward and backward direction. It is based on a tunable impedance surface, which has been described in previous patent applications, including the application that matured into U.S. Pat. No. 6,496,155 listed above. It is also based on a steerable leaky wave antenna, which has been described in previous patent applications, including the application that matured into U.S. Pat. No. 6,496,155 listed above. However, in the previous disclosures, it was not disclosed how to produce backward leaky wave radiation, and therefore the steering range of the antenna was limited. Furthermore, the presently described technology also provides new ways of improving the gain of leaky wave antennas.
A tunable impedance surface is shown in FIGS. 1(a) and 1(b) at numeral 10. It includes a lattice of small metal patches 12 printed on one side of a dielectric substrate 11, and a ground plane 16 printed on the other side of the dielectric substrate 11. Some (typically one-half) of the patches 12 are connected to the ground plane 16 through metal plated vias 14, while the remaining patches are connected by vias 18 to bias lines 18′ that are located on the other side of the ground plane 16, which vias 18 penetrate the ground plane 16 through apertures 22 therein. The patches 12 are each connected to their neighbors by varactor diodes 20.
In FIG. 1(a) the biased patches are easily identifiable since they are each associated with a metal plated vias 14 that penetrate the integral ground plane 16 through openings 22 in the ground plane, the openings 22 being indicated by dashed lines in FIG. 1(a). The ground patches are those that have no associated opening 22. The diodes 20 are arranged so that when a positive voltage is applied to the biased patches, the diodes 20 reverse-biased.
The return path that completes the circuit consists of the grounded patches that are coupled to the ground plane 16 by vias 14. The biased and grounded patches 12 are preferably arranged in a checkerboard pattern. While this technology preferably uses this particular embodiment of a tunable impedance surface as the preferred embodiment, other ways of making a tunable impedance surface can also be used. Specifically, any lattice of coupled and tunable oscillators could be used.
In one mode of operation that has previously been described in my aforementioned U.S. patent, this surface is used as an electronically steerable reflector, but that is not the subject of the present disclosure. In another mode of operation, the surface is used as a tunable substrate that supports leaky waves, which is the mode that is employed for this technology. This tuning technique has been the subject of other patent applications with both mechanically tuned and electrically tuned structures using a method referred to here as the “traditional method.” In a typical configuration using the “traditional method,” leaky waves are launched across the tunable surface 10 using a flared notch antenna 30, such as shown in FIG. 2. The flared notch antenna 30 excites a transverse electric (TE) wave 32, which travels across the surface. Under certain conditions, TE waves are leaky, which means that they radiate a portion of their energy 34 as they travel across the tunable surface 10. By tuning the surface 10, the angle at which the leaky waves radiate can be steered. All of the varactor diodes 20 are provided with the same bias voltage, so that the resonance frequency of each unit cell (a unit cell is defined by as a single patch 12 with one-half of each connected varactor diode 20 or equivalently as a single varactor diode 20 with one-half of each connected patch 12) changes by the same amount, and the surface impedance properties are uniform across the surface 10.
The traditional leaky wave beam steering method can be understood by examining the dispersion diagram shown in FIG. 3. The textured, tunable impedance surface 10 supports both TM and TE waves at different frequencies. TM waves are supported below the resonance frequency, denoted by ω1, and TE waves are supported above it. The “light line,” denoted by the diagonal line, represents electromagnetic waves moving in free space. All modes that lie below the light line are bound to the surface, and cannot radiate. See FIG. 4(a), which depicts phase matching when radiation is not possible for modes below the “light line.” The portion of the TE band that lies above the “light line,” on the other hand, corresponds to leaky waves 34 that radiate energy away from the surface 10 at an angle θ determined by phase matching, as shown in FIG. 4(b). Modes with wave vectors longer than the free space wavelength cannot radiate, while for shorter wave vectors, the angle of radiation is determined by phase matching at the surface. In the “traditional method,” the beam can only be steered in the forward direction where θ is greater than 0° and less than 90°.
The wave vector along the tunable impedance surface must match the tangential component of the radiated wave. The radiated beam can be steered in the elevation plane by tuning the resonance frequency from ω1 to ω2. When the surface resonance frequency is ω1, indicated by the solid line in FIG. 3, a wave launched across the surface at ωA will have wave vector k1. When the surface is tuned to ω2, as indicated by a dashed line in FIG. 3, the wave vector changes to k2, and the radiated beam is steered to a different angle. The beam angle q varies from near the horizon to near zenith as the resonance frequency is increased. In this traditional beam steering method, the entire surface is tuned uniformly. In actual practice, the radiated beam 32 can be steered over a range of roughly 5 degrees to 40 degrees from zenith, as shown in FIGS. 5(a)-5(e). FIGS. (a)-5(e) present graphs of measured results using the traditional leaky wave beam steering method with a uniform surface impedance obtained by applying the indicated DC voltages uniformly to all varactor diodes 20 in the electrically tunable surface 10. Radiation directly toward zenith or close to the horizon is not practical, and backward leaky wave radiation is not possible. Measurements were taken at 4.5 GHz for FIGS. 5(a)-5(e) with patch sizes of 0.9 cm disposed on 1.0 cm centers. The substrate 11 had a dielectric constant of 2.2, and was 62 mils (1.6 mm) thick. The varactor diodes 20 had an effective tuning range of 0.2 to 0.8 pF.